Conventionally, in feeding a liquid through a pipe having a narrow flow of area such as a pipe used in a hydraulic circuit or the like, when solubility of a gas dissolved in the liquid is high, there may be a case that air bubbles are generated in the liquid owing to a change of pressure applied to the liquid. These air bubbles are liable to easily obstruct the smooth flow of the liquid, may cause vibrations and noises or may give rise to a problem on smooth feeding of liquid or damages on the liquid by heat. Accordingly, in the hydraulic circuit or the like, an air bubble removal apparatus which removes air bubbles generated in the pipe is provided separately, or a liquid whose gas solubility is reduced by carrying out a deaerating process in advance is used.
On the other hand, in the food industry, as represented by recently popular “oxygen water”, in order to increase functionality of a liquid, a gas such as oxygen is dissolved in a liquid at concentration higher than normal concentration.
These kinds of deaerating process and gas dissolving process are, in general, conducted such that a liquid to be processed is contained in a container vessel capable of containing the liquid in an airtight manner. In the case of the deaerating process, the inside of the container vessel is depressurized down to a predetermined degree of vacuum or below, while in the case of the dissolving process, a gas to be dissolved in the liquid to be processed is filled in the inside of the container vessel under pressure to bring the inside of the container vessel into a predetermined pressurized state and this pressurized state is left for a predetermined time.
In particular, in the case of the dissolving process, by carrying out bubbling of the liquid to be processed with the gas to be dissolved, it is also possible to make the dissolving process require a shorter process time. Also, in the case of carrying out the dissolving process, it is desirable that the process of deaerating the liquid is carried out in advance so as to remove an unnecessary gas and, at the same time, to make it easier for a necessary gas to dissolve. The process of dissolving a predetermined gas in the liquid to be processed may also be called a gas impregnating process.
In the case of the deaerating process, in place of the above-mentioned batch type process, there has been proposed an apparatus which includes a vacuum tank depressurized in advance at a high degree of vacuum and a pipe which feeds the liquid to the vacuum tank and carries out a deaerating process by continuously feeding a liquid to be processed into the vacuum tank (for example, see Patent Document 1).
Also, as another apparatus for deaerating process, there has been proposed an apparatus which carries out a deaerating process in such a manner that microscopic air bubbles are produced in a liquid to be processed by generating cavitation in the liquid and the microscopic air bubbles are removed (for example, see Patent Document 2).
In such a circumstance, in the course of study on a cavitation phenomenon in a liquid, the inventor of the present invention have found that when supercavitation which is one form of the cavitation phenomenon occurs, a cavity is stably formed in the pipe through which the liquid is being fed (for example, see Non-patent Document 1).
Moreover, the inventor has found that the pressure inside the cavity formed inside the pipe by supercavitation is stably maintained at a very low pressure level close to the vapor pressure of the liquid, and thereby a gas dissolved in the liquid which is in contact with the cavity is separated into the cavity, that is, the liquid is deaerated by the cavity formed inside the pipe.
Furthermore, the inventor has connected the cavity maintained at a pressure considerably lower than an atmospheric pressure with the atmosphere, and has found a phenomenon that an internal pressure of the cavity rises to the atmospheric pressure while maintaining a stable shape without disappearing, air sucked into the cavity is mixed with the liquid and is formed into microscopic air bubbles, and these microscopic air bubbles are expelled in large quantities from a downstream end of the cavity. The inventor also has found a phenomenon that air is quickly dissolved into the liquid through this process of generation of microscopic air bubbles.
Patent Document 1: JP-A-07-132201
Patent Document 2: JP-A-03-118803
Non-patent document 1: Seiichi Washio, other 3, “Observation of Cavitation Inception in Hydraulic Oil Flow”, Japan Society of Mechanical Engineers collected papers, Japan Society of Mechanical Engineers, Inc., May, 1999, Volume 65, Number 633, Edition B, Pages 139 to 147